1. Field of the Invention
The invention relates to the extinction of the electric arc which occurs during breaking of a circuit by a switch device, under DC or AC operation.
It relates more particularly to low and medium voltage breaking apparatus (going for example from 110 V to 5 KV) in which the discharge occurs in the air and more especially those in which an electrically insulating screen is interposed between the contacts so as to promote rapid extinction of the arc and to prevent it from restriking.
These apparatus comprise: limiters where the separation of the contacts is for example obtained by means of electrodynamic repulsion forces which are exerted in conducting portions supporting the contacts when the current which flows through them exceeds a predetermined threshold, other types of circuit breakers where the separation of the contacts is obtained by releasing accumulated mechanical energy or by the magnetic energy developed by a short circuit and even some contactors.
2. Description of the Prior Art
In the Prior Art, the breaking technique principally used, in the above mentioned range, an extension effect of the arc.
Now, the arc voltage which is caused to increase as much as possible so as to cause extinction depends, not only on its length, but also on the electric field which is proportional to the intensity of the current and inversely proportional, on the one hand to the conductivity and on the other to the section of the arc. This latter factor has not been used up to now systematically with appropriate measures taken for controlling it so as to increase the arc voltage.
The applicant has discovered that a much more rapid extinction of the arc is obtained if, all other things being equal, a practically complete shearing of the arc is effected between the screen and an electrically insulating wall with which it cooperates, on the express condition that the screen moves at a sufficient speed for continually destabilizing the arc from the moment when it occurs to the effective moment of shearing, and that the breaking chamber is adapted so as to offer no leak path for the arc, outside the path along which the destabilization takes place. When these conditions are fulfilled, the arc voltage increases practically as quickly as in a fuse.
According to the patent DT No. 84 9138, filed on July 8, 1949, two pivoting shutters pivot back towards a central wall with which they come into contact so as to interpose an obstacle in the paths of the arc between two respective pairs of contacts. However, after separation, the arrangement is such that the shutters take up their pivoted back position in a time equal to the time for opening the contacts, which is moreover relatively long, because this opening is controlled by mechanical means. The arc may therefore stabilize itself and it is neither indicated, nor probable, that its extinction is caused by the subsequent interposition of the screen, rather than by the zero cross-over of the current. This device seems in fact intended for preventing the arc from restriking; before extinction of the arc, this latter is in contact with the screen for a relatively long time, which results in a considerable risk of metallization of the screen by the arc. Furthermore, constriction is not completely provided and leak paths are not avoided; finally, the whole of the structure is neither reliable, nor capable of providing efficient operation for a long period of time.
According to the published patent application DT No. 1010618, filed on Jan. 21, 1952, the screen undergoing a translational movement perpendicular to the direction of separation of the contacts, is only inserted between the contact pieces after these latter have been moved away from each other by the cooperation, with their support blades, of bosses with which it is provided. The means for opening the contacts is here united with the means for propelling the screen and this latter must completely overcome the pressure containing the contacts in the closed condition. It follows necessarily that the interposition in the path of the arc of the screen thus braked in its translational movement occurs after a relatively long time, so that the arc will have time to stabilize itself. A blowout coil is moreover disposed in the vicinity of the contact pieces for urging the arc thus formed towards a slit into which the edge of the screen penetrates. Stifling of the arc only takes place after it has undergone considerable lengthening and the whole device is housed in a very resistant sealed chamber so that an overpressure is created; these two factors (lengthening and overpressure) contribute to increasing the arc voltage: in so far as the stifling is concerned, it is neither indicated, nor probable that it is complete and that it forms an appreciable factor contributing to the extinction.